As is known from the prior art, amongst the various functions performed by a relay, the main one is that of detecting failures and faults and of controlling opening of the circuit by the circuit-breaker. Amongst the possible failures there exist, in particular, initial instantaneous short circuits, characterized by currents Iinst, which can cause enormous and irreversible damage even in periods of time in the region of a few milliseconds. By “initial instantaneous short-circuit”, hereinafter referred to as SCinst, is meant a short-circuit already present at the moment of, or concomitant with, closing of the circuit by the circuit-breaker. Said short circuits are particularly critical since they require the circuit-breaker and the relay to carry out opening immediately, as soon as the circuit is closed.
It should, however, be recalled that, as soon as the circuit-breaker is in the closing condition, for the relay to be able to function properly it is necessary to wait a short but significant start-up time, in which the electrical and electronic parts are subject to a transient that brings them up to steady-state conditions. Once this time has elapsed, the relay is able to perform the normal functions of protection and to control opening of the circuit by the circuit-breaker.
One of the data characterizing relays is hence the start-up time Tsu of the relays themselves.
The start-up time Tsu depends upon different factors, listed below:                power-supply start-up time (Tps) of the supply device of the relay defined above, said supply device having as input the stretch of electrical network protected by the circuit-breaker and as output a supply adequate for the characteristics of the relay;        characteristic time of stabilization of the hardware components of the relay (Thw) necessary for operation of the microcontroller (for example, the oscillator);        characteristic time of initialization of the software (Tsw) present in the microcontroller;        time for calculating the currents (Tc), i.e., the time necessary for processing the signal coming from the current sensors in a form useful for generating the signal for actuation of the protections.        
Neglecting any partial possible superposition, it may be said that the start-up time Tsu is given by the sum of these partial times, namely, Tsu=Tps+Thw+Tsw+Tc. A circuit-breaker that does not envisage any additional strategy, during the time Tsu is consequently virtually unable to provide any protection. Hence, any failure or fault that may occur in the part of the system protected by the circuit-breaker during this time Tsu cannot be adequately interpreted by the relay.
In the known art, progressively more rapid systems and components have been studied and introduced, capable of reducing the time Tsu to values in the region of 15 ms. At the current state of the art, just the reduction of Tsu is therefore insufficient to set a circuit-breaker in conditions of safety in the case of instantaneous short circuits. In fact, in the case of instantaneous short circuits (SCinst), tripping is desirable in times in the region of 2 ms.
Other systems of the known art solve the problem of protection of instantaneous short circuits (SCinst) by providing alongside the main protection device, other additional protection devices, which are very fast in the initial transient and are capable of tripping with marked anticipation with respect to the main protection device. The additional protection devices adopted in the known art for performing the protection function SCinst are based upon one of the two following types of solutions:
i) magnetomechanical solutions, obtained for example with sensors and actuators set in the proximity of the conductors; and
ii) electronic solutions with simple components (made, for example, with passive comparators), which do not envisage a microcontroller and are thus subject Tps alone.
The additional protection devices based upon these two solutions are generally far inferior, from the standpoint of precision and the possibility of calibration, to the main protection device, but in comparison with the latter they present the advantage of reacting in much shorter time intervals. They are normally calibrated in a fixed way so that they trip at current levels that are certainly dangerous, namely when the lack of interruption of the main circuit in times shorter than the time Tsu defined above is likely to expose the stretch of electrical network protected by the circuit-breaker or the circuit-breaker itself to permanent damage.